The invention relates to the preparation of metal oxide catalysts that are useful in the oxidation of propane.
Thousands of tons of acrylic acid and acrylonitrile are produced from propylene each year. The lower price of propane relative to propylene made the development of a process starting with propane very attractive. Accordingly, a MoVTeNb mixed-metal oxide catalyst that gives good yields of acrylonitrile and acrylic acid from propane was developed and has been the subject of a significant amount of research.
In a typical catalyst synthesis, water-soluble metal precursor compounds are dissolved in water, and the resulting mixture is dried to form a solid precursor mixture of metal compounds. The precursor is heated to an intermediate temperature in air to decompose and drive off water, ammonia, and organics, and then is heated under an inert (oxygen-free) atmosphere to a higher temperature to form the mixed-metal oxide catalyst.
The Mo—V—Te—Nb—O system is complex, with multiple phases kinetically accessible under the conditions required to prepare the best catalysts. An orthorhombic phase, called M1 in the literature, has the Cs0.7(Nb2.7W2.3)O14, structure. This phase is generally acknowledged to be necessary to activate propane. A pseudo-hexagonal phase, called M2, has a modified hexagonal tungsten bronze (HTB) structure. In addition to these, many preparations also include an undesirable M5O14 phase. An intergrowth tungsten bronze (ITB) phase having the formula TeM5O16 is sometimes seen as well. Thus, it is difficult to prepare a material having only an M1 phase on a commercial scale.
In view of the unsettled state of the art, it would be desirable to have a improved process for preparing a MoVTeNb mixed-metal oxide catalyst.